The present invention relates to turbine power plants; and more particularly, to an improved system for controlling the dynamic operation of turbines automatically without operator intervention.
Turbine power systems typically include a high pressure (HP) turbine section where the steam is introduced directly from the steam generator. The steam from the HP turbine section after being reheated is introduced into a reheat turbine section, which in the case of fossil-fired steam generating systems is commonly termed the (IP) turbine section; and then into a low pressure turbine section before exhausting to the condenser. A rotor having an axial bore passes centrally through the turbine casings; and rotation of the rotor is achieved by passage of the steam over blades alternately affixed to the rotor and to the casing. The generator, which is affixed to the rotor, may be cooled by hydrogen has (H.sub.2).
The rotor of the HP turbine section may be typically in the order of 24 inches in diameter, for example, and the IP turbine section, includes a rotor which may be in the order of over forty inches in diameter. The IP rotor surface is replete with grooves and other irregularities, particularly where the turbine blades are affixed.
It is well known, that whenever the turbine is to undergo changes in speed, and the generator is to undergo changes in load, care must be taken lest damage be done either by thermal stresses, thermal expansion of adjacent parts of different rates, or by exceeding the capability of the generator. A turbine which undergoes thermal stress caused by uneven heat distribution in the rotors, tends to develop cracks at locations on the rotor most exposed to the widest and most frequent steam temperature variation. Also, such cracks will occur when the turbine is accelerated at too fast a rate when the turbine rotors are not of uniform temperatures.
The present invention is an improvement over the prior art system, as disclosed in U.S. patent application Ser. No. 408,962, which is a continuation of Ser. No. 247,887, filed by Theodore C. Giras and Robert Uran, on Apr. 26, 1972, entitled "System And Method For Starting, Synchronizing, And Operating A Steam Turbine With Digital Computer Control" and assigned to the assignee hereof, which is, in its entirety, hereby incorporated into the present application by reference. This referenced application, which discloses an automatic system for starting up a turbine includes certain details which form one part of the invention of the present system including the features of the related referenced applications (1) through (4) and shall be referred to hereinafter as the Giras application.
The Giras application includes an automatic start-up system for steam turbine power plant which controls the turbine under the thermal constraints of HP rotor stress from rolling off turning gear to synchronous speed, and the application of initial load. The system monitors plant conditions to inform the operation of dangerous conditions after the application of initial load. The Giras start-up system recognizes that the IP rotor is considered the most critical for speeds above the heat soak speed of approximately two-thirds synchronous speed when the rotor temperature is below 250.degree. F. The rotor metal is in a brittle state below 250.degree. F. which may result in the development of cracks in the event of excessive speeds.
In the Giras system, the turbine is prevented from exceeding the heat soak speed for a period of time based upon a time versus temperature curve, which must be conservatively estimated in order to protect the turbine. Specifically, the computation of this heat soak time, or time versus temperature curve, is based conservatively on the lowest of four calculated temperatures. A comparison is made between the calculated (1) the rotor volume average temperature which existed before opening the steam inlet valves, (2) the rotor volume average temperature at 2200 rpm's, (3) the first stage turbine metal temperature before opening the steam inlet valves. (4) and the first stage metal temperature at 2200 rpm's. When the heat soak speed has been reached, the amount of heat soak time is determined, based upon the lowest temperature selected from the above for a reheat steam temperature of 500.degree. F. Once the soak time is completed, a final check on the HP rotor volume average temperature is made before declaring that the heat soak is complete and allows the turbine to continue acceleration. In the event that the lowest of these temperatures is above 250.degree. F., the heat soak is considered unnecessary.
After the predetermined heat soak time is completed, the system accelerates the turbine to approximately 3300 rpm's at a rate which is determined by a calculated HP rotor strain which is compared to a selected rotor strain limit. After the system automatically transfers from throttle to governor valve control at 3300 rpm's the turbine is accelerated to synchronous speed. After the application of a minimum load, the system is supervisory only, that is, various parameters are monitored and appropriate messages are printed to assist the operator in the control of the turbine up to the desired load.
In the Giras application, the HP rotor surface thermal strain is proportional to the surface-to-volume average temperature differential and determines the acceleration of the turbine. A comparison of the present thermal strain value with previous thermal strain values determines the type of thermal transient that the rotor is undergoing, and selects the proper acceleration path to be followed. The rotor surface temperature is calculated as a function of the first stage HP steam temperature, the present heat transfer coefficient, and the history of the temperature of the rotor metal. The magnitude of the rotor strain is determined by the surface-to-volume average rotor temperature which is utilized to determine the rotor surface strain based on present and past history. The heat transfer coefficient is computed as a function of speed reaching its higher value in the speed mode at rated speed.
The system of the Giras application is advantageous in so far as it rotates to start-up of the turbine through the application of initial load; however, the heat soaking of the critical IP turbine rotor is based on a time versus temperature curve, which may result in an unnecessary elapsed time. With such elapsed heat soak time consecutively estimated, the HP rotor stress calculations provided sufficient thermal stress protection for automatic operation up to synchronous speed.
With respect to the calculation of HP rotor strain and various means for controlling the turbine in accordance with such strain, reference is made to U.S. Pat. No. 3,448,265, entitled "System And Method For Providing Steam Turbine Operation With Improved Dynamics", by William R. Berry, and assigned to the present assignee, in which there is discussed in detail the effects of thermal loading on permissible turbine operation, which is incorporated by reference herein for the purpose of indicating the background of certain aspects of the present invention. The referenced patent to Berry discloses an improved method of determining present rotor stress as a function of monitored HP turbine impulse chamber steam temperature, comparing the present stress with a predetermined stress limit, and deriving a control signal from such comparison, by which inlet steam to the HP turbine is controlled. In such a prior art system, the impulse chamber steam pressure at the HP turbine section may be further controlled by considerations of rotor bore loading or casing strain. The effects of thermal expansion and contraction of respective regions of the turbine are thus controlled as a function of calculated stress at such regions, which calculations are based upon the monitored inlet steam condition, centrifugal force loadings, and other input variables.
The Berry patent teaches that bore thermal stress calculations can be made for the reheat turbine by determining the rotor surface temperature in the inlet steam region of the reheat pressure section based upon the measured reheat inlet chamber steam temperature and the variable and lower heat transfer conductance of the reheat rotor surface in the same manner as the HP turbine.
Berry suggests that on-line rotor bore loading determinations can be eliminated in the event that a predetermined heat soak time is utilized in the start-up procedure. Berry mentions that the heat transfer conductance of the IP turbine is further determined as a predetermined function of the IP steam flow and IP steam density or pressure; that is EQU K.sub.(IS).sub..sbsb.i.sbsb.p = (W.sub.s SF, P.sub.IP)
where W.sub.s = actual turbine speed, SF= IP steam flow, and P.sub.IP = IP steam pressure, and K.sub.(IS).sub..sbsb.i.sbsb.p is the heat transfer conductance of the IP rotor.
Another specific prior art example of turbine operation based upon considerations of rotor stress is disclosed in a patent to Zwicky, U.S. Pat. No. 3,446,224, issued May 27, 1969. This patent calculates rotor bore and surface stresses by means of temperature and speed measurements; and calculates safe stress margins, and applies the lowest of the surface or bore safe stress margin as either an acceleration reference signal or a load rate reference signal to control the acceleration and load of the turbine. Calculations of bore stress and bore temperature are made by periodically taking the inner casing steam temperature at three consecutive time intervals and multiplying by predetermined constants. Only the time intervals are varied according to the diameter of the rotor. In Moore, U.S. Pat. No. 3,561,216 issued Feb. 9, 1971, there is disclosed a rotor stress controlled system which calculates rotor stress in the same manner as the patent to Zwicky. In this patent, the rate of loading and the single-to-sequential transfer of the valves is governed by the highest stress of all the calculated thermal stresses. U.S. Pat. No. 3,577,733 issued to Manuel on May 4, 1971 discloses a method of loading a steam turbine and transferring between partial arc and full arc steam admission modes during loading while maintaining a constant rate of heating.
In each of the prior art examples, different systems are disclosed for preventing either cyclic variations in the temperature of the turbine rotors or for calculating rotor stress in order that a turbine may be operated without undue thermal strain. These patents recognize that the greatest thermal differences occur in the high pressure rotor because of differentials in steam temperature and small diameter of the rotor; and the patents to Berry and Zwicky suggest that such stress can be calculated with respect to the reheat turbine rotor as well as the high pressure turbine rotor by taking a longer time for heat conductance.
An automatic turbine control system which controls the turbine without operator intervention up to application of a desired operator entered load must be efficient in its operation; and take into consideration any undesirable conditions of operation that would tend to shorten the life of the component parts of the plant. In so doing the system should have versatility such that the undesirable conditions can be prevented, or rectified without interrupting turbine operation. In furtherance thereof, it is desirable that the system can increase or decrease the rate of loading in accordance with such conditions up to an operator entered medium.
The thermal stress of the rotors, both HP and IP should be considered for such a system, as well as the constraints of the electric generator. Also, such a system should control in real-time through all phases of its operation, with proper predictions of what will occur in the event the system is controlling the plant at a certain rate of increased load.
In determining the thermal stress of the IP turbine rotor, such a system should provide for the critical stress points that exist axially along the rotor as well as provide for different stresses for different types of blade mountings.